The disclosure relates generally to equalizers used for equalizing radio frequency (RF) signals and more particularly to a tunable RF equalizer which may be used in communications equipment such as a distributed antenna system (DAS).
An equalizer is a circuit that attempts to reverse the distortion that a system introduces into a signal as it passes through a communication channel. A variety of communication systems make use of equalizers to improve signal performance. For example, narrow band RF slope equalizers are widely used in various DAS applications to reduce signal distortion.
More specifically, a typical DAS may include complicated designs that require many components. For example, amplifiers, filters, interface units and the like may be included within the DAS. The combinations of components may cause perturbations in the power of a RF signal (which may also be referred to as “ripple” discussed further below). Accordingly, various designs for DASs make use of equalizers to reduce ripple in the RF signals.
There are several topologies for equalizers that are commonly used today. All of the topologies incorporate a frequency resonance close to the band of operation. Generally, the resonance frequency is configured such that if the band of operation requires a negative slope of equalizing, then the resonance frequency of the equalizer is located just above the band of operation frequency range. If the band of operation requires a positive slope of equalizing, then the resonance frequency of the equalizer is located just below the cellular band of operation frequency range.
One example of a prior art equalizer 10 is shown in FIG. 1. This embodiment of the prior art equalizer 10 makes use of a varactor diode as a tunable capacitor. Unfortunately, the prior art equalizer 10 exhibits high temperature variations due to the nature of the varactor diode. Accordingly, this requires calibration and preferably adjustment of output according to temperature through temperature monitoring and additional control. Additionally, a high degree of variation during production of the varactor diode requires calibration, thus adding expensive complications to the design of the prior art equalizer 10. Further, the prior art equalizer 10 exhibits a low degree of linearity (low third-order intercept point, OIP3) due to the nature of the varactor diode. For example, a typical prior art equalizer 10 would exhibit an OIP3 value of about 35-40 dBm, thus practically eliminating use in high power nodes of the DAS.
The foregoing third-order intercept point (OIP3) previously mentioned is one measure for judging the linearity of weakly nonlinear systems and devices, for example receivers, linear amplifiers, and mixers. The OIP3 is based on the idea that device nonlinearity can be modeled using a low-order polynomial, derived by means of Taylor series expansion. The third-order intercept point relates nonlinear products caused by the third-order nonlinear term to the linearly amplified signal, in contrast to the second-order intercept point that uses second-order terms. As indicated, the prior art equalizer 10 exhibits a low degree of linearity (low third-order intercept point, OIP3) due to the nature of the varactor diode.
Of course, other embodiments of prior art equalizers 10 are known. Unfortunately, the other embodiments likewise are constrained and inadequate for various reasons. For example, some other embodiments of prior art equalizers 10 make use of a single digitally tunable capacitor (DTC) instead of a varactor diode. The problems with DTCs are that they exhibit a rather small capacitance tuning range (which can be very problematic when having to correct for both temperature and production) as well as a relatively low self-resonance frequency (SRF) which is caused by a resonance between the DTC maximum capacitance and self-inductance of the DTC. Typically, the SRF of the DTC should be at least 1.5 times the upper edge of the operating equalized band. Otherwise, there will be an uncontrolled notch frequency close to the equalized operating band, making equalizers based upon DTCs useless at high operating frequencies (for example, up about 2 GHz).
What are needed are improved methods and apparatus to provide for equalization of the RF signal in a communication system. The methods and apparatus should provide consistent quality devices that are useful over a wide range of radio frequencies, and stable over an operational temperature range.
No admission is made that any reference cited herein constitutes prior art. Applicant expressly reserves the right to challenge the accuracy and pertinency of any cited documents.